Endometriosis is a common gynecologic disease affecting up to 10% of reproductive-age women. Despite this high prevalence and the severe symptoms associated with the disease, little is known about the pathogenesis of endometriosis. One theory, known as Sampson's theory, proposes that fragments of menstrual endometrium pass retrograde through the fallopian tubes into the peritoneal cavity where they attach and grow on peritoneal surfaces. We recently developed a novel in vitro model of endometriosis using explants of human peritoneum or mesothelial cell monolayers and mechanically dispersed endometrial cells. Our studies demonstrate that endometrial fragments rapidly adhere to intact cultured peritoneal mesothelium. Both ESC and EEC adhere to peritoneal mesothelium within one hour of plating. Recent studies suggest that hyaluronan, a linear disaccharides polymer produced by mesothelium, and CD44, a multifunctional type 1 transmembrane glycoprotein that regulates cell-cell interactions, are involved in the binding of ovarian cancer and gastric cancer cells to mesothelium. Using our model, we demonstrated that hyaluronidase inhibits attachment of endometrial cells to mesothelial cells suggesting that hyaluronan/CD44 is also involved in the pathogenesis of endometriosis. A significant body of evidence using cell types other than endometrial suggests that the CD44 isoform expression, CD44 cell surface density, and CD44 glycosylation/glycosaminoglycanation pattern differentially affect a cells ability to adhere to hyaluronan. Our preliminary data demonstrate that endometrial epithelial cells from women with endometriosis have a greater ability to bind to mesothelial cells and that binding to mesothelial cells is dependent on the cell surface density of CD44. These observations coupled with the variable expression of CD44 isoforms in human endometrium lead us to hypothesize that the qualitative and quantitative expression of CD44 regulates the ability of endometrial cells to adhere to peritoneal mesothelium. The novel experiments described herein will characterize CD44 cell isoform expression, cell surface density and glycosylation/glycosaminoglycanation patterns in endometrial cells of women with and without endometriosis. This will enhance our understanding of the development of the early endometriotic lesion. The findings should enable us to predict a woman's risk of developing endometriosis based on endometrial cell CD44 characteristics and suggest new approaches to prevent the disease.